1. Field of the Invention:
This invention relates to a method for continuous production of a rubber-reinforced polystyrene type resin possessing a balanced set of outstanding properties.
Rubber-reinforced polystyrene, for example, is now used extensively in parts for household electric appliances. It nevertheless has the disadvantage that the molded articles thereof are deficient in surface gloss, in impact strength, and in colored feeling as compared with those of the conventional ABS resin. Recently, in view of the trend of the molded articles of rubber-reinforced polystyrene type resins toward reduction of cost and decrease of wall thickness, the market has been demanding the appearance of a rubber-reinforced polystyrene composition which exhibits properties similar to those of the ABS resin.
As the general trend of the recent techniques concerning the production of rubber-reinforced polystyrene, the method of continuous bulk polymerization which gives products of highly stable quality at low cost has come to attract increasing attention in view of reduction of cost.
This invention aims to provide a rubber-reinforced polystyrene type resin composition which is inexpensive and yet is capable of producing molded articles possessing a balanced set of outstanding properties such as gloss, coloring, and impact strength comparable to those exhibited by injection molded articles, extruded sheets, and extrusion vacuum molded articles of the expensive ABS resin and a method for the continuous production, particularly a method for the continuous bulk polymerization, of the rubber-reinforced polystyrene type resin composition.
2. Description of the Prior Art:
The rubber-reinforced polystyrene has heretofore been used extensively on a commercial scale. The rubber particles dispersed in the rubber-reinforced polystyrene have diameters generally falling in the range of 1.0 to 5.0 microns (.mu.). When the rubber-reinforced polystyrene contains rubber particles of diameters not exceeding 1.0 .mu., it does not exhibit desirable properties.
The rubber-reinforced polystyrene generally has polystyrene included in the form of minute particles within the dispersed particles of rubbery substance. It is clearly noted from the electron microscope photographs of commercially available rubber-reinforced polystyrene products inserted as FIG. 2 in "Encyclopedia of Polymer Science and Technology," Vol. 13, page 402 that in all the included polystyrene particles of a given rubber-reinforced polystyrene product, those of diameters exceeding 0.3 .mu. account for a fairly large proportion.
Resin compositions which comprise a rubber-reinforced styrene type resin and an organic polysiloxane are described in such prior publications as "Modern Plastics," November, 1972 issue, pp 114-116, "Plastics Age," 1974, Vol. 20, May issue, page 107, Japanese Patent Application Laid-open No. 3494/1980, Japanese Patent Application Laid-open No. 124561/1978, Japanese Patent Application Laid-open No. 187345/1982, and Japanese Patent Application Laid-open No. 187346/1982. From the contents of these publications, it is clear that a desirable resin composition is not obtained when the rubber-reinforced polystyrene component used therein contains rubber particles of smaller diameters.
To meet the aforementioned market's demand for an improved rubber-reinforced polystyrene possessing a balanced set of outstanding properties, it is necessary to offer an improved rubber-reinforced polystyrene which exhibits outstanding and balanced performance in impact strength, gloss, tensile strength, and heat resistance.
The method which comprises polymerizing a styrene monomer solution containing a rubbery substance thereby making particles of the rubbery substance, bulk polymerizing the resulting composite until the produced particles of rubbery substance acquire stable diameters, and switching the proceeding bulk polymerization to suspension polymerization has been known to the art. The method which adds polystyrene before start of polymerization (Japanese Patent Publication No. 19352/1966, U.S. Pat. No. 3,488,743), the method which adds polystyrene while the polymerization is in progress and then continues the polymerization to make particles of the rubbery substance (Japanese Patent Publication No. 13983/1968, Japanese Patent Publication No. 21746/1968), and the method which polymerizes part of a styrene solution of the rubbery substance and then adds an additional supply of rubbery substane and optionally a styrene type polymer to make particles of the rubbery substance (Japanese Patent Publication No. 35074/1974) have been also known to the art. These methods invariably include a step of suspension polymerization. Besides, the method which adds polystyrene before the polymerization is started in either bulk-suspension polymerization or bulk polymerization (U.S. Pat. No. 3,144,420, for example) has been known to the art. Further, the method which mixes the rubbery substance particles with a separately prepared polystyrene solution and continues the polymerization without a catalyst (U.S. Pat. No. 3,676,527, U.S. Pat. No. 2,694,692) have been proposed.
It is, however, difficult to obtain a rubber-reinforced polystyrene type resin which produces molded articles of excellent appearance (gloss and coloring). It is far more difficult to obtain this resin by a method of continuous bulk polymerization. None of the methods of continuous bulk polymerization developed to date has succeeded in obtaining this resin with satisfactory performance.